When a closure is extracted as a method, it does not have any return type.

pkg/analysis_server/lib/src/generated_protocol.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // This file has been automatically generated.  Please do not edit it manually.
 // To regenerate the file, use the script
 // "pkg/analysis_server/tool/spec/generate_files".
 
 part of protocol;
 /**
@@ skipping 10520 matching lines @@
    * extracted.
    */
   int offset;
 
   /**
    * The length of the expression or statements that will be extracted.
    */
   int length;
 
   /**
-   * The proposed return type for the method.
+   * The proposed return type for the method. If the returned element does not
+   * have a declared return type, this field will contain an empty string.
    */
   String returnType;
 
   /**
    * The proposed names for the method.
    */
   List<String> names;
 
   /**
    * True if a getter could be created rather than a method.
@@ skipping 754 matching lines @@
     return false;
   }
 
   @override
   int get hashCode {
     int hash = 0;
     hash = _JenkinsSmiHash.combine(hash, newName.hashCode);
     return _JenkinsSmiHash.finish(hash);
   }
 }